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Johannes Kepler (1571-1630), German astronomer. Kepler devised the three fundamental laws of planetary motion. These laws were based on detailed observations of the planets made by himself and the Danish astronomer Tycho Brahe. Kepler's first law states that the planets orbit the Sun in elliptical paths, with the Sun at one focus of the ellipse. The second law states that the closer a planet comes to the Sun, the faster it moves. Kepler's third law states that the ratio of the cube of a planet's mean distance from the Sun to the square of its orbital period is a constant. Newton used these ideas to formulate his theory of gravity.